Perimeter intrusion detection system

ABSTRACT

A perimeter intrusion detection system includes an elongated magnetic core member with a helical winding positioned about the magnetic core and a source of AC current applied to the helical winding. The combination of the magnetic core and helical winding is positioned adjacent to or below the surface of the earth about the perimeter of an object to be protected i.e., a building, an airplane, a military encampment, etc. Monitoring circuitry is attached to the helical winding and responds to a change in the earth&#39;&#39;s magnetic field produced by the presence of a magnetically permeable object by generating an intrusion alarm signal. The sensitivity of the monitoring circuitry can be adjusted so as to produce an alarm signal in response to the presence of magnetically permeable material carried on the person of an intruder or magnetically permeable material associated with a vehicle approaching the protected object. The invention makes use of the theory of operation of a flux gate magnetometer to provide a continuous perimeter intrusion detection device capable of monitoring several hundred feet of perimeter and providing indication of the presence of a magnetically permeable object at a specific interval of the permimeter intrusion detection system.

United States Patent 1 Miller et al.

[ PER IMETER INTRUSION DETECTION SYSTEM Inventors: Robert C; Miller;Walter J. Carr,

Jr., both of Pittsburgh, Pa.

Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

Filed: Oct. 10, 1972 Appl. No.: 296,376

US. Cl...... 340/258 C, 235/92 TC, 340/38 L Int. Cl. G08b 13/22 Field ofSearch 340/258 C, 258 D, 38 L;

[56] References Cited UNlTED STATES PATENTS 8/1973 Shtrikman 340/258 C3/1973 Kardashian 340/38 L l2/l959 Mierendorf et al 324/41 PrimaryExaminer-John W. Caldwell Assistant'ExaminerGlen R. Swann, lll Attorney,Agent, or Firm-M. P. Lynch:

[57] ABSTRACT A perimeter intrusion detection system includes an ACEXCITATION ac MAGNETIC FIELD MEAS CIRCUIT [11] 3,812,484 May 21, 1974elongated magnetic core member with-a helical winding positioned aboutthe magnetic core and a source of AC current applied to the helicalwinding. The combination of the magnetic core and helical winding ispositioned adjacent to or below the surface of the earth about theperimeter of an object to be protected i.e., a building, an airplane, amilitary encampment, etc. Monitoring circuitry is attached to thehelical winding and responds to a change in the earths magnetic fieldproduced by the presence of a magnetically permeable object bygenerating an intrusion alarm signal. The sensitivity of the monitoringcircuitry can be adjusted so as to produce an alarm signal in responseto the presence of magnetically permeable material carried on the personof an intruder or magnetically permeable material associated with avehicle approaching the protected object. The invention makes use of thetheory of operation of a flux gate magnetometer to provide a continuousperimeter intrusion detection device capable of monitoring severalhundred feet of perimeter and providing indication of the presence of amagnetically permeable object at a specific interval of the permimeterintrusion detection system.

5 Claims, 5 Drawing Figures aATENTEDHAY21 m4 EXCITATION SHEET 1 BF 2 DC.MAGNETIC FIELD MEAS CIRCUIT FIG. 3

PATENTEU IIAY 2 I 1974 SNEET 2 0F 21 AC. EXCITATION v FILTER 1 km 2CIRCUIT Q T DETECTION CIRCUIT FILTER 42 CIRCUIT I Q DETECTION CIRCUITA.C. EXClTATlON FILTER CIRCUrr 59 1' FIG. 4

DETECTION J4 CIRCUIT STEPPING SWITCH 72 74 R i n76 7,3

2 HARMONIC I DIFFERENTIAL AL RM i AMPLIFIER EEUR I CIRCUIT l iCOMPARATOR so DRIVE 3 CIRCUIT -7| 7T TREND PERIMETER INTRUSION DETECTIONSYSTEM BACKGROUND OF THE INVENTION There exists an ever increasingdemand for security apparatus. A most serious security requirement isfor detection systems capable of monitoring the movement of personneland vehicles and particularly the move- SUMMARY OF THE INVENTION Theperimeter intrusion detection system described herein with reference tothe accompanying drawings is a perimeter device for detecting intrudersor vehicles on the basis of the presence of magnetically permeableobjects which produce a distrubance in the earths magnetic field.Inasmuch as most weapons, and guns in particular, are constructed frommagnetically permeable material, a system capable of responding to thepresence of such material provides an important security tool. Asensitive measurement of the earths field is obtained from asystemincluding a ferromagnetic core magnetized to saturation by a windingcarrying alternating current and monitoring circuitry for measuring thesecond harmonic of the signal induced in the winding. This secondharmonic is a measure of the DC field. The perimeter intrusion detectionsystem described herein includes a long continuous core member which maybe several feet or several hundred feet in length. The core can be afine wire or a'thin ribbon of magnetically permeable material, such as aferromagnetic material typically represented herein as Permalloy. Asource of AC current is applied to a continuous helical winding which iswound about the elongated core to magnetize the core to saturation.Monitoring circuitry associated with the helical winding responds to thesecond harmonic of the signal induced in the winding as a result of thepresence of a magnetically permeable material adjacent to the perimeterintrusion detection system. An alarm indication is produced when thesignal is indicative of a class of objects of interest, i.e., weapons.

Several embodiments of the invention are disclosed and described toprovide various degrees of sophistication in monitoring the perimeter ofa protected object. While the simplest embodiment utilizes a singlehelical coil, the preferred embodiment utilizes a secondary coiloperatively coupled to the elongated core to develop the signalindicative of the presence of a magnetically permeable object.Additional system accuracy is obtained by applying a bucking current tothe system in order to cancel out the effect of the unperturbed earthsmagnetic field on the signal induced in the secondary winding.

Yet another variation of the invention utilizes numerous tap locationsalong the secondary winding in order to develop output signalsindicative of specific intervals of the perimeter intrusion detectionsystem in order to not only detect intrusion along the perimeterrepresented by the intrusion detection system but to further isolate theintrusion to a specific interval or intervals of the perimeter intrusiondetection system.

DESCRIPTION OF. THE DRAWING The invention will become more readilyapparent from the following exemplary description in connection with theaccompanying drawings:

FIG. 1 is a schematic illustration of an embodiment of the invention;

FIG. 2 is an electrical schematic of a basic configuration of the systemof FIG. 1;

FIG. 3 is an alternate electrical schematic illustration of the systemof FIG. 1;

FIG. 4 is an electrical schematic illustration of a particular windingarrangement of the secondary winding 7 of the embodiment of FIG. 1; and

FIG. 5 is an alternate schematic illustration of a tapped secondarywinding for the embodiment of FIG.

I DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isillustrated schematically a perimeter intrusion detection system 10comprised of four intrusion detection devices 20 positioned about theperimeter of a protected area PA herein represented as including abuilding in a substantially parallel relationship with the surface ofthe ground G. The intrusion detection device 20 is shown in threedifferent installations, one with the device buried beneath the surfaceof the ground, a second with the device 20 positioned on the surface ofthe ground and a third configuration with the device 20 secured to afence F. Inasmuch as each of the perimeter intrusion detection devicesshown in FIG. 1 is substantially identical a description will be limitedto the embodiment shown buried beneath the surface of the ground.

The intrusion detection device 20 is comprised ofan elongated coremember 22 and a helical coil element 24 wound thereabout. The coreelement may be in the form of a tine wire or a thin ribbon of amagnetically permeablematerial, such as soft iron. The combination ofthe core member 22 and the helical coil element 24 is positioned withinhousing 26 which functions to isolate the core and helical coil fromdamaging contact resulting from adverse weather conditions. A typicalembodiment of the intrusion detection device 20 consists of a coremember of wire approximately 3/32 inches in diameter and 10 to feet inlength in a helical coil of wire wherein the diameter of the wire isapproximately 0.040 inches. Experimental analysis has shown that lengthsvarying from 10 to several hundred feet of continuous core and coilmaterial will provide suitable signals for determining the intrusion ofa perimeter by a magnetically permeable object.

An excitation circuit 30 comprised of an AC source is connected to thehelical coil element 24 of the intrusion detection device 20. Theexcitation circuit 30 functions to'apply an alternating current to thehelical coil element 24 sufficient to saturate core member 22.Monitoring circuitry 40 is attached to the helical coil element 24 tomonitor the second harmonic content of the signal induced in the helicalcoil element 24 in response to the presence of a magnetically permeableobject in the vicinity of the intrusion detection device 20. Monitoringcircuit 40 is set to respond to a predetermined level of the secondharmonic signal which is representative of a predetermined class ofmagnetically permeable objects. In the application of a perimeterinperimeter of the protected area PA are illustrated as having separateAC excitation sources, it is apparent that a single AC excitation sourcecould be utilized to provide saturation of the core members of thevarious intrusion detection devices. Furthermore, the four separateintrusion detection devices of FIG. 1 could be replaced by a singleintrusion detection device having a core and helical coil of sufficientlength to extend about the perimeter of the protected area PA thusnecessitating but one AC excitation source 30 and one monitoring circuit40.

While the embodiment shown in FIG. 1 depicts the basic operation of theperimeter intrusion detection system 10, variations of this embodimentcan be designed to eliminate the effect of changes in the earthsmagnetic field produced by remote conditions unrelated to the presenceof magnetic permeable objects and to further provide identification of aparticular interval or intervals of the perimeter intrusion detectionsystem within which an intrusion is taking place.

Referring to FlG. 2, there is schematically illustrated the basicembodiment of the intrusion detection device of FIG. 1 including avariable DC source V a capacitor C and an inductor L connected to thehelical coil element 24. The function of the voltage source V toestablish a bucking or nulling current of sufficient value toessentially eliminate the effect of the unperturbed earths magneticfield on the signal transmitted to the monitoring circuit 40 in responseto the presence of a magnetically permeable object in the vicinity ofthe intruding detection device 20. The level of the nulling current isestablished such that in the absence of an in trusion no second harmonicsignal is transmitted to the detector circuit 44. Under theseconditions, the small change in the magnetic field resulting from anintrusion is more readily detected. The capacitors C and inductors Lfunction to provide paths for the AC and DC signals while isolating theAC and DC circuits.

The monitoring circuit 40 is shown as including a filter member 42 toeffectively reject the fundamental of the voltage induced in the helicalcoil element 24 and a second harmonic detection circuit 44 for producingan output signal indicative of the level of the second harmonic signaldeveloped in the helical coil element 24. The second harmonic detectorcircuit 44 may be implemented through the use of a phase detectorcircuit such as model HR8 for Princeton Applied Research. 7' I While asingle winding configuration provides a workable embodiment, there isillustrated in FIG. 3 an embodiment of the intrusion detection devicecomprised of a primary winding 52, a secondary winding 54 and a buckingwinding 56. The AC excitation source 30, typically an oscillatorcircuit, is connected-to the primary winding 52 and the bucking voltagesource V,,,. is connected to the bucking winding 56. The combination ofthe filter circuit 42 and the second harmonic detection circuit 44 isconnected to the secondary winding 54 and operates in a manner asdescribed with reference to FIG. 2.

There is schematically illustrated in FIG. 4 a secondary winding 60 ofthe intrusion detection device illustrated in FIG. 3 wherein thesecondary winding is comprised of a plurality of winding elements 60serially connected in alternately opposed relationship. This serialarrangement of alternately opposed segments of the secondary windingfunctions to cancel the effect of changes in the earths magnetic fieldcaused by remote conditions i.e., lightning, etc. This cancellationresults from the fact that the oppositely connected adjacent segments ofwinding produce identical induced signals but of opposite polarity.

The secondary winding illustrated in FIG. 5 includes a plurality ofspaced apart tap locations T. The induced signals produced in thesecondary winding in the intervals defined by the adjacent tap locationsT are transmitted by lead wires 62 to a sequential scanning mechanism,herein illustrated to be a stepping switch 70. The stepping switch isactivated by drive circuit 71 to sequentially scan adjacent pairs ofintervals of the secondary winding and applies the induced signalspresent in adjacent segments to differential amplifier 72.'A differencein the adjacent signals indicative of variation in the earths magneticfield in the respective intervals, as would be caused by the presence ofa magnetically permeable object in the vicinity of one of the intervals,re-. sults in an output signal from the differential amplifier 72 to thesecond harmonic detection circuit 74. The second harmonic detectioncircuit 74 supplies an output signal proportional to the difference inthe second harmonic signal content of the output signal from thedifferential amplifier 72 to the comparator circuit 76. Comparatorcircuit 76 compares the inputfrom the second harmonic detection circuit74 to a reference R representative of the second harmonic signalattributable to predetermined class of magnetically permeable objects ofinterest i.e., weapons, and if the input from a second harmonicdetection circuit 74 corresponds to the preselected class of objects thecomparator circuit 76 activates alarm circuit 78.

The output signals from the comparator circuit 76 are also supplied asinputs'to the trend recorder 80 which sequentially plots the'intrusioninformation thus providing an indication as to the number of secondarycoils segments affected by the intrusion of a magnetically permeableobject, this plot represents information as to the size of themagnetically permeable object.

We claim: I

1. In a perimeter intrusion detection apparatus for providing anindication of the presence of a magnetically permeable object, thecombination of, an elon gated magnetic core member and coil means woundthereabout, said combination of the core member and coil means beingpositioned relative to the surface of the earth to respond to changes inthe earths magnetic field, an AC excitation source operatively connectedto said coil means for establishing a current flow in said coil means tosaturate said core member, said coil means responding to changes in theearths magnetic field caused by the presence of a magnetically permeableobject by developing AC output signals, and first circuit meansoperatively connected to said coil means for responding to the contentof said AC output signals indicative of the strength of the magneticfield.

2. In a perimeter intrusion detection apparatus as claimed in claim 1,wherein said coil means consists of a primary helical winding woundabout said core mem- I bcr and a secondary helical winding wound aboutsaid core member, said AC excitation source operatively connected tosaid primary winding and said circuit means operatively connected tosaid secondary winding, said secondary winding comprised of a pluralityof intervals, said circuit means monitoring the AC signals developed bythe respective intervals to determine the interval or intervals closestto the magnetically permeable objects.

3. In a perimeter intrusion detection apparatus as claimed in claim 2wherein said coil means includes a third helical winding wound aboutsaid core member and second circuit means operatively connected theretoto apply current of a polarity and magnitude sufficient to cancel theeffect of the earths magnetic field.

4. In a perimeter intrusion apparatus as claimed in claim 1 wherein saidfirst circuit means includes a second harmonic detection circuit, thesecond harmonic content of said AC output signals being a measure of thestrength of the DC magnetic field, said first circuit means develping anoutput signal indicative of the second harmonic content of said ACoutput signals.

5. In a perimeter intrusion detection apparatus as claimed in claim 4further including third circuit means operatively connected to saidfirst circuit means for comparing the output signal of said firstcircuit means to a reference signal indicative of the strength of amagnetic field representative of a predetermined magnetically permeableobject, said third circuit means generating an output signal when theoutput signal of said first circuit means corresponds to said referencesignal.

1. In a perimeter intrusion detection apparatus for providing anindication of the presence of a magnetically permeable object, thecombination of, an elongated magnetic core member and coil means woundthereabout, said combination of the core member and coil means beingpositioned relative to the surface of the earth to respond to changes inthe earth''s magnetic field, an AC excitation source operativelyconnected to said coil means for establishing a current flow in saidcoil means to saturate said core member, said coil means responding tochanges in the earth''s magnetic field caused by the presence of amagnetically permeable object by developing AC output signals, and firstcircuit means operatively connected to said coil means for responding tothe content of said AC output signals indicative of the strength of themagnetic field.
 2. In a perimeter intrusion detection apparatus asclaimed in claim 1, wherein said coil means consists of a primaryhelical winding wound about said core member and a secondary helicalwinding wound about said core member, said AC excitation sourceoperatively connected to said primary winding and said circuit meansoperatively connected to said secondary winding, said secondary windingcomprised of a plurality of intervals, said circuit means monitoring theAC signals developed by the respective intervals to determine theinterval or intervals closest to the magnetically permeable objects. 3.In a perimeter intrusion detection apparatus as claimed in claim 2wherein said coil means includes a third helical winding wouNd aboutsaid core member and second circuit means operatively connected theretoto apply current of a polarity and magnitude sufficient to cancel theeffect of the earth''s magnetic field.
 4. In a perimeter intrusionapparatus as claimed in claim 1 wherein said first circuit meansincludes a second harmonic detection circuit, the second harmoniccontent of said AC output signals being a measure of the strength of theDC magnetic field, said first circuit means develping an output signalindicative of the second harmonic content of said AC output signals. 5.In a perimeter intrusion detection apparatus as claimed in claim 4further including third circuit means operatively connected to saidfirst circuit means for comparing the output signal of said firstcircuit means to a reference signal indicative of the strength of amagnetic field representative of a predetermined magnetically permeableobject, said third circuit means generating an output signal when theoutput signal of said first circuit means corresponds to said referencesignal.